TheAlgorithms/Java · error · IllegalArgumentException

Input number cannot be negative

Error message

Input number cannot be negative

What it means

Thrown by Factorial.factorial when n < 0. Factorial is defined for non-negative integers; the iterative loop `for (i = 1; i <= n; i++)` would not execute for negative n and would silently return 1 (wrong), so the library rejects negatives explicitly to avoid a silently incorrect result.

Source

Thrown at src/main/java/com/thealgorithms/maths/Factorial.java:11

package com.thealgorithms.maths;

import java.math.BigInteger;

public final class Factorial {
    private Factorial() {
    }

    public static BigInteger factorial(int n) {
        if (n < 0) {
            throw new IllegalArgumentException("Input number cannot be negative");
        }
        BigInteger result = BigInteger.ONE;
        for (int i = 1; i <= n; i++) {
            result = result.multiply(BigInteger.valueOf(i));
        }
        return result;
    }
}

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Pass a non-negative integer n (>= 0); factorial(0) correctly returns 1.
2. Guard at the caller: if (n < 0) reject or clamp to 0.
3. Validate parsed input before invoking.

Example fix

// before
BigInteger f = Factorial.factorial(count - 1); // count == 0 => -1

// after
BigInteger f = Factorial.factorial(Math.max(0, count - 1));

if (n < 0) {
    throw new IllegalArgumentException("factorial requires n >= 0");
}
Factorial.factorial(n);

static boolean isNonNegative(int n) { return n >= 0; }

When it happens

Trigger: Calling factorial(-1) or any negative n. Common when n is derived from a subtraction or parsed from unvalidated input.

Common situations: n computed as a - b that can go negative; user input not bounded; loop boundaries that include 0 or below; off-by-one in decrementing logic.

Related errors

• Must be a natural number
• number is negative
• Input must be a positive integer.
• Input must me positive.
• n must be positive and odd.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/46c3af7f6ab75f3c. Report an issue: GitHub.